The present invention relates to a process and apparatus for continuously producing a polyester. As is known in the art, continuous production of polyesters is customarily carried out in three stages. These are the esterification or interesterification stage, the precondensation stage, and the polycondensation stage. The present invention is particularly concerned with an improvement in the esterification or interesterification.
More specifically, the present invention is concerned with a process and apparatus for the continuous production of a polyester by means of the esterification of an aromatic dicarboxylic acid, for example, terephthalic acid, with an alkylene alcohol, for example, ethylene glycol, or by means of the interesterification of a diester of a dicarboxylic acid, for example, dimethyl terephthalate, with an alkylene alcohol, for example, ethylene glycol, to form a low molecular weight bis-.beta.-hydroxyethylene terephthalate, precondensation under reduced pressure to form a higher molecular weight ethylene terephthalate, and polycondensation to form an even higher molecular weight polyethylene terephthalate (polyester), which, if desired, can be further processed.
The chemical reaction of an alkylene alcohol, particularly ethylene glycol (EG), with an aromatic dicarboxylic acid, for example, terephthalic acid (TPA), is an esterification reaction (direct esterification). The chemical reaction of a diester of a dicarboxylic acid, for example, dimethyl terephthalate, with an alkylene alcohol, particularly with ethylene glycol, is an interesterification reaction. The final product of both reactions is a low molecular weight bis-.beta.-hydroxyethylene terephthalate (BHET).
In practice, the BHET is transformed into a higher molecular weight product in a precondensation stage under reduced pressure at an esterification degree of approximately 95% , and is finally polycondensed into an even higher molecular weight polyethylene terephthalate (polyester) in the polycondensation stage under a further reduced pressure, which permits further processing.
While, in the past, the production of polyester was mainly based on dimethyl terephthalate and ethylene glycol, terephthalic acid is mostly used today. However, there are problems with the direct esterification presently in practice, which are mainly due to the fact that terephthalic acid actually dissolves in bis-.beta.-hydroxyethylene terephthalate, but practically does not dissolve in ethylene glycol, which results in a reduction in the esterification velocity. This negative effect is emphasized by the comparatively low reactivity of terephthalic acid with ethylene glycol, and this leads to undesirably long reaction times, which not only increases the complexity of the apparatus required and thus reduces the economy of the process, but also impairs the product quality.
There have been attempts to overcome this disadvantage of direct esterification by shortening the reaction time through varying the process parameters, for example, increasing the process temperature and such. However, there are limits for such measures which are determined by the product quality. Thus, for example, the side reactions increase with a rise in temperature, which leads to, among other substances, diethylene glycol (DEG), and this reduces the product quality.